Thin film transistors (“TFT's”) are widely used as switching elements in electronics, for example, in active-matrix liquid-crystal displays, smart cards, and a variety of other electronic devices and components thereof. The thin film transistor (TFT) is an example of a field effect transistor (“FET”). The best-known example of an FET is the “MOSFET (Metal-Oxide-Semiconductor-FET) that can be used for high-speed applications. Most thin film devices are made using amorphous silicon as the semiconductor because amorphous silicon is a less expensive alternative to crystalline silicon. This fact is especially important for reducing the cost of transistors in large-area applications. Application of amorphous silicon is limited to low speed devices, however, since its maximum mobility (0.5-1.0 cm2/V·sec) is about a thousand times smaller than that of crystalline silicon.
The use of amorphous silicon has its drawbacks. The deposition of amorphous silicon, during the manufacture of transistors, requires relatively costly processes, such as plasma enhanced chemical vapor deposition and high temperatures (about 360° C.) to achieve the electrical characteristics sufficient for display applications. Such high processing temperatures disallow the use of substrates, for deposition, made of certain plastics that might otherwise be desirable for use in applications such as flexible displays.
More recently, organic materials have received attention as a potential alternative to amorphous silicon for use in semiconductor channels of TFT's. Organic semiconductor materials are simpler to process, especially those that are soluble in organic solvents and, therefore, capable of being applied to large areas by far less expensive processes, such as spin-coating, dip-coating and microcontact printing. Furthermore, organic materials can be deposited at lower temperatures, opening up a wider range of substrate materials, including plastics, for flexible electronic devices. Accordingly, thin film transistors made of organic materials can be viewed as a potential key technology for plastic circuitry or devices where ease of fabrication or moderate operating temperatures are important considerations or mechanical flexibility of the product is desired.
Organic semiconductor materials can be used in TFT's to provide the switching or logic elements in electronic components, many of which require significant mobilities, well above 0.01 cm2/V·sec, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000. Organic TFT's having such properties are capable of use for electronic applications such as pixel drivers for displays, identification tags, portable computers, pagers, memory elements in transaction carts, and electronic signs. Organic materials for use as potential semiconductor channels in TFT's are disclosed, for example, in U.S. Pat. No. 5,347,144 (Garnier et al.).
Considerable efforts have been made to discover new organic semiconductor materials that can be used in FET's to provide switching or logic elements in electronic components, many of which require significant mobilities well above 0.01 cm2/V·sec, and current on/off ratios (hereinafter referred to as “on/off ratios”) greater than 1000. Organic FET's (“OFET's”) having such properties can be used for electronic applications such as pixel drivers for displays and identification tags. Most of the compounds exhibiting these desirable properties are “p-type” or “p-channel,” however, meaning that negative gate voltages, relative to the source voltage, are applied to induce positive charges (holes) in the channel region of the device.
As an alternative to p-type organic semiconductor materials, n-type organic semiconductor materials can be used in FET's where the terminology “n-type” or “n-channel” indicates that positive gate voltages, relative to the source voltage, are applied to induce negative charges in the channel region of the device.
Moreover, one important type of FET circuit, known as a complementary circuit, requires an n-type semiconductor material in addition to a p-type semiconductor material. In particular, the fabrication of complementary circuits requires at least one p-channel FET and at least one n-channel FET. Simple components such as inverters have been realized using complementary circuit architecture. Advantages of complementary circuits, relative to ordinary FET circuits, include lower power dissipation, longer lifetime, and better tolerance of noise. In such complementary circuits, it is often desirable to have the mobility and the on/off ratio of an n-channel device similar in magnitude to the mobility and the on/off ratio of a p-channel device. Hybrid complementary circuits using an organic p-type semiconductor and an inorganic n-type semiconductor are known, but for ease of fabrication, an organic n-channel semiconductor material would be desired in such circuits.
Only a limited number of organic materials have been developed for use as a semiconductor n-channel in OFET's. One such material, buckminsterfullerene C60, exhibits a mobility of 0.08 cm2/V·sec but it is considered unstable in air (Haddon et al. Appl. Phys. Let. 1995, 67, 121). Perfluorinated copper phthalocyanine has a mobility of 0.03 cm2/V·sec and is generally stable to air operation, but substrates must be heated to temperatures above 100° C. in order to maximize the mobility in this material (Bao et al. Am. Chem., Soc. 1998, 120, 207). Other n-channel semiconductors, including some based on a naphthalene framework, have also been reported, but with lower mobilities. One such naphthalene-based n-channel semiconductor material, tetracyanonaphthoquino-dimethane (TCNNQD), is capable of operation in air, but the material has displayed a low on/off ratio and is also difficult to prepare and purify.
Aromatic tetracarboxylic diimides, based on a naphthalene aromatic framework, have also been demonstrated to provide, as an n-type semiconductor, n-channel mobilities up to 0.16 cm2/V·sec using top-contact configured devices where the source and drain electrodes are on top of the semiconductor. Comparable results could be obtained with bottom contact devices, that is, where the source and drain electrodes are underneath the semiconductor, but a thiol underlayer must then be applied between the electrodes (that must be gold) and the semiconductor as described in U.S. Pat. No. 6,387,727 (Katz et al.). In the absence of the thiol underlayer, the mobility of these compounds was found to be orders of magnitude lower in bottom-contact devices. This patent also discloses fused-ring tetracarboxylic diimide compounds, one example of which is N,N′-bis(4-trifluoromethyl benzyl)naphthalene diimide. The highest mobilities of 0.1 to 0.2 cm2/V·sec were reported for N,N′-dioctyl naphthalene diimide.
In a different study, using pulse-radiolysis time-resolved microwave conductivity measurements, relatively high mobilities have been measured in films of naphthalene diimides having linear alkyl side chains (Struijk et al., J. Am. Chem. Soc. Vol. 2000, 122, 11057).
U.S. Patent Application Publication 2002/0164835 (Dimitrakopoulos et al.) discloses n-channel semiconductor films made from perylene diimide compounds, as compared to naphthalene-based compounds, one example of which is N,N′-di(n-1H,1H-perfluorooctyl) perylene diimide. Substituents attached to the imide nitrogens in the diimide structure comprise alkyl chains, electron deficient alkyl groups, and electron deficient benzyl groups, and the chains preferably having a length of four to eighteen atoms. Devices based on materials having a perylene framework used as the organic semiconductor have low mobilities, for example 10−5 cm2/V·sec for perylene tetracarboxylic dianhydride (PTCDA) and 1.5×10−5 cm2/V·sec for N,N′-diphenyl perylene diimide (PTCDI-Ph) (Horowitz et al. Adv. Mater. 1996, 8, 242 and Ostrick et al. J. Appl. Phys. 1997, 81, 6804).
The morphology of an organic film has a strong impact on the charge transport and overall device performance of organic thin film transistors. In general, the morphology of organic films depends directly on the chemical structure of the molecules that controls the interaction between the molecules. In crystalline organic films defects, like grain boundaries and disorder inside the grains, are limiting factors for the mobility and the diffusion length of the charge carriers. The extent of it-stacking between the molecules determines whether the organic film will be highly crystalline or totally amorphous independently of other growth controlling parameters like the substrate and its temperature.
In perylene and naphthalene diimide based OFET's, many experimental studies have demonstrated that morphology of the thin film has strong impact on the device performances. Theoretical calculation and experimental characterization (particularly X-ray diffraction), have shown that the molecular packing in PDI is very sensitive to the side chains (Kazmaier et al. J. Am. Chem. Soc. 1994, 116, 9684). In perylene diimide based n-channel OFET devices, changing the side chain from n-pentyl to n-octyl increases the field effect mobility of from 0.055 cm2/V·sec to 1.3 cm2/V·sec, respectively (Chesterfield et al. J. Phys. Chem. B 2004, 108, 19281). Such sensitivity to the type of side-chain is a manifestation of an aggregation effect and it provides potentially an effective way to control and optimize the molecular packing for enhanced π-orbital overlap between neighboring molecules, a necessary for efficient carrier transport. U.S. Pat. No. 7,422,777 (Shukla et al.) discloses N,N′-dicycloalkyl-substituted naphthalene diimide compounds, which in thin films, exhibit optimum packing and exhibit n-channel mobility up to 6 cm2/V·sec in OFET's. U.S. Pat. No. 7,579,619 (Shukla et al.) discloses N,N′-di(arylalkyl) substituted naphthalene diimide compounds that exhibit high n-channel mobility up to 3 cm2/V·sec in top-contact OFET's.
U.S. Patent Application Publications 2008/0135833 (Shukla et al.) and 2009/0256137 (Shukla et al.) describe n-type semiconductor materials for thin film transistors that include configurationally controlled N,N′-dicycloalkyl-substituted naphthalene 1,4,5,8-bis-carboximide compounds or N,N′-1,4,5,8-naphthalenetetracarboxylic acid imides having a fluorinated substituent, respectively.
A variety of naphthalene diimides have been made and tested for n-type semiconducting properties. In general, these materials, as an n-type semiconductor, have provided n-channel mobilities up to 6 cm2/V·sec using top-contact configured devices. However, besides charge mobility, improved stability and integrity of the semiconductor layer is an important goal.
A way to improve organic semiconductor layer stability and integrity in a device would be to include the organic semiconductor molecule in a polymeric additive. However, the performance of OFET's, characterized by parameters such as the field effect mobility and threshold voltage, depends in part upon the molecular structure and crystalline order of the semiconductor film. Structure and molecular ordering of the semiconductor film depends in turn on how the thin film is deposited. It is generally believed that increasing the amount of molecular order by increasing crystal size, reducing the density of crystalline defects, or improving short-range molecular order, permits charge carriers, that is, electrons or holes, to more efficiently move between molecules. This can increase the field effect mobility.
U.S. Patent Application Publication 2005/0176970 (Marks et al.) discloses improved n-channel semiconductor films made of mono and diimide perylene and naphthalene compounds, nitrogen and core substituted with electron withdrawing groups. Substituents attached to the imide nitrogens in the diimide structure can be selected from alkyl, cycloalkyl, substituted cycloalkyl, aryl and substituted aryl moieties. However, this publication fails to suggest any comparative advantage of using cycloalkyl groups on the imide nitrogens. Accordingly, mobilities obtained from perylene diimides containing of N-octyl and N-cyclohexyl are virtually indistinguishable (Example 10). Furthermore, the highest mobilities reported in this reference were 0.2 cm2/V·sec and it fails to show experimental data with respect to naphthalene compounds and require that their core be dicyano disubstituted.
Aromatic tetracarboxylic diimides, based on a naphthalene and perylene aromatic framework have been widely used as n-type semiconductor materials (Newman et al. Chem. Mater. 2004, 16, 4436-4451). Relatively low mobilities have been measured in films of naphthalene tetracarboxylic diimides having linear alkyl side chains using pulse-radiolysis time-resolved microwave conductivity measurements. See Struijk et al. “Liquid Crystalline Perylene Diimides: Architecture and Charge Carrier Mobilities” J. Am. Chem. Soc. Vol. 2000, 122, 11057. However, TFT's based on N,N′-dicyclo substituted naphthalene diimide exhibit mobility up to 5 cm2/V·sec (Shukla et al. Chem. Mater. 2008, 20, 7486-7491). U.S. Pat. No. 6,387,727 (Katz et al.) discloses fused-ring tetracarboxylic diimide compounds, such as N,N′-bis(4-trifluoromethyl benzyl)naphthalene-1,4,5,8,-tetracarboxylic acid diimide. The highest mobilities reported in this patent is between 0.1 and 0.2 cm2/V·sec, for N,N′-dioctyl naphthalene-1,4,5,8-tetracarboxylic acid diimide.
Higher carrier mobilities are obtained when naphthalene tetracarboxylic diimides are substituted with aryl groups. Accordingly, U.S. Pat. No. 7,629,605 (Shukla et al.) discloses a thin film of organic semiconductor material that comprises an N,N′-diaryl-substituted naphthalene-based tetracarboxylic diimide compound having a substituted or unsubstituted aromatic ring system directly attached to each imide nitrogen atom, wherein the substituents on at least one or both of the aromatic ring systems comprises at least one electron donating organic group. These materials consistently exhibit higher mobility compared to a naphthalene tetracarboxylic diimide having phenyl substituents.
As is clear from the foregoing discussion, the development of new semiconducting materials, both p-type and n-type, continues to be an enormous topic of interest and unpredictable as to the semiconductive properties of various compounds. Among n-type diimide based materials, the highest charge carrier mobility (ca. 5.0 cm2/V·sec) in thin film transistors has been observed with N,N′-dicyclohexyl-naphthalene diimide. However, the poor solubility of this material limits its practical application potential.
Recently, dicyanated arylene diimide semiconductors based on perylene and naphthalene diimide cores have been developed that are solution processable and show environmental stability (Adv. Funct. Mater. 2008, 18, 1329-1339). The latter characteristics arise from cyano group addition to the core, which increases solubility by decreasing molecular planarity and stabilizes charge carriers by lowering the energies of the lowest unoccupied molecular orbital's associated with electron transport. While high temperature vapor deposited devices using these materials show good mobilities (ca. 0.1-0.5 cm2/V·sec; Jones et al. Adv. Funct. Mater. 2008, 18, 1329-1339), solution coated device usually give lower mobility and exhibit low Ion/Ioff ratio.
It is widely recognized that the morphology and microstructure of an organic thin film has a strong impact on the charge carrier mobility and OTFT device characteristics. In general, organic materials that form highly oriented polycrystalline thin films exhibit high charge carrier mobility. At the molecular level, it is the basic chemical structure of the molecule that controls intermolecular interactions that determines if a material will be crystalline or amorphous. Thus, to have well-defined thin film morphology it is necessary to control materials on the molecular scale. This necessitates adapting the basic structure of semiconducting molecules in a way that results in an optimum crystalline packing motif. In the case of diimide based n-type semiconducting materials to attain solubility extensive molecular modification have to be made which usually lowers the crystallinity of the material (for example see et al. Adv. Funct. Mater. 2008, 18, 1329-1339) that usually results in lower mobility in OTFT devices.
There remains a critical need in the art for improved performance of n-type organic semiconductor materials in OTFT's and improved technology for their manufacture and use. Specifically there is a need for materials and processes to obtain solution processable diimide based n-type semiconducting materials that do not require significant structural modification to achieve processability and optimum crystalline packing.
Amic acids are usually more soluble in solvents than the aromatic anhydrides that they are derived from. One attractive way of obtaining diimide based semiconductors in thin solid films is to solution coat amic acid and then by a thermal dehydration reaction, convert the amic acid to the corresponding diimide.
The dehydration of amic acids, derived from the reaction of cyclic anhydrides with primary amines, to yield imides is a general method for the preparation of this important class of heterocyclic compounds and is of major commercial significance in the conversion of polyamic acids to polyimides (J. A. Kreuz, A. L. Endrey, F. P. Gay, and C. E. Sroog, J. Polym. Sci., Part A, 4, 2607 (1966), and references contained therein.). As polyimides derived from phthalamic acids possess many desirable attributes, this class of materials have found uses in many technologies including dielectrics in microelectronics, high temperature adhesives, and membranes (for example, see K. L. Mittal, Polyimides and Other High Temperature Polymers: Synthesis, Characterization and Applications vol. 1 to 5). Most of the detailed studies have concentrated on the preparation of polyphthalamic acids and their conversion to polyimides in solid films (for example see Kim et al. in Polymer 40, 1999, pp 2263-2270, and references cited therein). In contrast, little is known about the dehydration reactions of amic acids derived from naphthalene and perylene anhydrides or naphthalene and perylene tetracarboxylic acid dianhydrides. Fabienne et al. have recently reported mechanistic studies of polycondensation reactions of naphthalene anhydride leading to naphthalimide polymers (Piroux, Fabienne; Mercier, Regis; Picq, Dominique, High Performance Polymers (2009), 21(5), 624-632). However, these authors do not disclose amic acids and amic esters of naphthalene tetracarboxylic acids or anhydrides.
Genies et al. have reported synthesis of soluble sulfonated naphthalenic polyimides, derived from naphthalene dianhydride, as materials for proton exchange membranes (Genies et al., Polymer 42 (2001) 359-373). However, these authors do not disclose the preparation of amic acid or amic ester from naphthalene dianhydride.